1. Field
This embodiment relates to an annealing method of a zinc oxide thin film.
2. Description of the Related Art
Zinc oxide (ZnO) is used for a transparent conducting oxide (TCO), an emitter, nano quantum dot array, a nanorod, a nanobelt and the like in photoelectric devices such as a thin film solar cell, a thin film transistor liquid crystal display (TFT LCD), an organic electro luminescence device (OELD), a blue light emitting diode (LED) and a laser diode (LD).
The zinc oxide is a material which has n-type conduction by electrons because the Fermi level of the zinc oxide is located near the conduction band even though the zinc oxide is a nominally undoped thin film.
In a process of manufacturing a zinc oxide thin film used in a thin film solar cell, a TFT LCD, an OELD, an LED or the like, a wet cleansing process is frequently performed to protect the zinc oxide thin film from impurities.
Accordingly, moisture may remain in the zinc oxide thin film after the wet cleansing process. In this case, the remaining moisture (H2O) may be easily absorbed on a grain boundary or surface of the zinc oxide thin film.
Moreover, when the zinc oxide thin film is formed using a chemical vapor deposition (CVD) process, H2O is used as a raw material gas. Therefore, H2O which is not reacted during the formation of the zinc oxide thin film may remain on the grain boundary or surface of the zinc oxide thin film.
As described above, H2O may remain in the zinc oxide thin film during the formation of the zinc oxide thin film or the wet cleansing process. In this case, when another thin film is deposited on the zinc oxide thin film, adhesion between the zinc oxide thin film and the another thin film is lowered, and therefore, the another thin film may be peeled off from the zinc oxide thin film.
Particularly, in a thin film solar cell, if moisture (H2O) remaining in a transparent electrode formed of a zinc oxide thin film is diffused in a silicon layer, an open-circuit voltage is lowered, and therefore, conversion efficiency of the thin film solar cell may be degraded. Accordingly, it is very important to remove the moisture (H2O) contained in the zinc oxide thin film for the purpose of manufacturing a thin film solar cell with high efficiency and high yield.
In an OELD, an organic film layer constituting a light emitting layer is deteriorated due to moisture (H2O) remaining in a zinc oxide thin film, so that the lifetime of the OELD may be reduced or light emitting efficiency may be degraded. Therefore, it is very important to remove the moisture (H2O) remaining in the zinc oxide thin film of the OELD for the purposed of improving characteristics of the OELD.
To remove moisture remaining in a zinc oxide thin film, a glass substrate on which a zinc oxide thin film is formed is loaded into a vacuum chamber, and the zinc oxide thin film is then annealed under a high vacuum state or an atmosphere of an inert gas such as nitrogen (N2) or argon (Ar), thereby removing moisture (H2O). However, the inert gas such as nitrogen (N2) or argon (Ar) is not diffused at a relatively high speed in the vacuum chamber, and therefore, processing time may be delayed.
Further, the inert gas such as nitrogen (N2) or argon (Ar) has a large molecular weight, and hence, it is not easy for the inert gas to be diffused through a grain boundary of the zinc oxide thin film. Therefore, the inert gas may not be effective to remove moisture absorbed on the grain boundary of the zinc oxide thin film.
Furthermore, the inert gas such as nitrogen (N2) or argon (Ar) does not serve as a dopant of the zinc oxide thin film, and therefore, electrical characteristics of the zinc oxide thin film may not be improved.